The present invention is generally directed to the manufacture and utilization of information storing systems employing rotatable storage devices which contain a reference track which renders it possible to perform multiple storage and retrieval functions including the identification of individual bit locations on a recordable disk. More particularly, the present invention is directed to an apparatus for monitoring maximum and minimum values of signals produced by a photodetector which receives laser light from the reference track edges. Even more particularly, the present invention is directed to an apparatus for monitoring semiconductor laser aging and for compensating for this aging with adjustments to the power input to the laser.
A central aspect of the present invention involves the recognition of a frequency doubling phenomenon that occurs when a light source having an essentially gaussian intensity distribution is dithered across a surface exhibiting a sharp reflectivity discontinuity (or other optical property discontinuity). The present invention exploits frequency doubling and phase reversal as mechanisms for accurately positioning a laser light source which is used to write information onto a recordable, rotatable blank in systems which can be made to be compatible with existing CD-ROM devices which unfortunately only perform read functions. While there do exist apparatus for performing disk-writing operations, these systems are expensive and cannot provide the information density which is rendered possible by the use of a servo tracking system of the kind described herein.
Clearly, the commercial craving for increased information density on recordable media has been a significant driving force in present day information-handling systems technology development. Moreover, commercial and market forces have increased the demand for a significant increase in the raw amount of data needed to be stored and accessed upon a removable device. These forces have also increased the demand that this information be available quickly and accurately. Moreover, there has been an increase in the desire to make the information on these disks writable by end users.
Other related developments in this technology have included the design and construction of units that are capable of writing information to several overlying layers in a single compact rotatable disk device. Furthermore, the desire to store graphic and moving picture images and sound on these devices has also produced variants of the now ubiquitous computer and audio CD-ROM devices and the market has now branched out into the production of DVD (digital video disk) platters. Accordingly, it is an object of the present invention to provide solutions to problems which occur in this emerging technology.
The present invention provides and is an overall solution to the problems and challenges produced by digital information storage technologies. The present invention provides a comprehensive and detailed solution to many of the problems existing in this industry. It is seen in the present invention that the inventors have started from a technology base related to servo positioning and have extended that capability and have utilized it to provide an extensive and comprehensive solution to all of the above-described problems of data storage, transmission speed and data accuracy. A complete end-to-end solution of the problems described is provided herein. It is therefore seen that the present inventive system begins with the production of information storage blanks which are employed in applicants' inventive system. Associated with these blanks, there is also included an apparatus for reading and writing to these disks. An additional beneficial mechanism that falls out from this work is a system for immediately performing read-after-write operations. This is possible since the signals produced by the servo tracking mechanism of the present invention are particularly useful for exceedingly accurate position control. Additionally, deliberate dithering and its signal consequences are employed to control focus for reading and writing multi-layer disks. Further improvements also result from the fact that laser beam splitting is employed to write information to both sides of a disk using simple and relatively inexpensive beam splitting devices. In all of these variations, applicants have been able to exploit the signals that are produced using specifically designed detector circuits for performing a variety of different functions. In particular, the notions and problems associated with error correction take on an entirely new meaning and perspective when performed within the context of the system claimed herein.
The detailed description below includes a complete characterization of all aspects of making and using the specific system or method claimed herein. Various aspects of this description can, however, be found as being described under the following general topics; accordingly, the detailed description is divided into several sections in which the following specific aspects are discussed: (I) Recording Blanks; (II) Basic Writing Operations; (III) Signal Detection; (IV) Reading after Writing; (V) Error Handling; (VI) Multi-layer Processing; and (VII) Read/Write Head Design.
To fully understand the advantages provided by the present invention, one should appreciate the problems presently found in CD-ROM writing technology. A significant problem in this area is the fact that the disks for writing are not inexpensive. Furthermore, a fundamental problem in writing to CD-ROM-like devices using a laser is that there is a definite need for data formatting. In present day CD-ROM writing technologies, all of the data medium looks the same. There are no reference points per se. Efforts at resolving this problem by providing molded grooves in a disk as a starting point are disadvantageous in that they are large and consume a significant amount of space on the disk. Other approaches which attempt to solve this problem employ master disks. These disks, however, require expensive mastering systems and the tracks are still too large or at least larger than needed. Therefore, it is seen that a significant need in the CD-ROM writing technology is a mechanism for providing inexpensive writable media.
Other problems in this area are also presented. In particular, it is known that there are, in fact, different data formats employed. For example, one data format may be employed for an audio CD-ROM, another format may be employed for a CD-ROM which stores computer program and data. Yet another data format may be employed for the storage of real-time video presentations such as is seen in the recently announced DVD technology (DVD stands variously either for Digital Video Disk or Digital Video Data).
Furthermore, in the midst of the technology explosion related to CD-ROM type devices, various implementations of multi-layer disks are being proposed.
Yet another problem with any advanced CD-ROM writing technology is the question of whether or not it is compatible with already existing CD-ROM reading technology. Furthermore, still further questions arise with respect to whether or not future technologies such as is provided by the present invention are compatible with systems such as magneto optical (MO) recording techniques. Fortunately, the system provided by the present invention is compatible with many existing technologies and provides break through approach in which ultimately each bit on a CD-ROM is addressable and locatable. As a consequence, the problems associated with disk formatting disappear. Effectively, such problems are relegated to software, leaving the hardware free to easily express information in a variety of formats.